Just finished playing around on it, and I must say, it's looking pretty good so far.
I really like the geometry of the level, you really feel like you're in a system of tunnels. But I did have a few problems with it.
As scorpio said, it is a tad small, which doesn't have to be a bad thing, but becuase of its perfect semetry, it is a bad thing. There needs to be a greater variation of areas, rooms, and hallways in the map. Right now, it all looks the same, and couple that with the fact that there's a perfect mirror between the left and right sides of the map, and you have navigation problems. You may not see this when you're playing by yourself, but in the heat of battle between 2 full teams, you're going to get turned around. To help prevent this, use more signs to indicate which team goes were.
Speaking of signs, going into a team's base on the top route, there is a sign that says flag room. This particular sign is actually not one that should be used in a final map. Valve included a bunch of "indicators" that mappers can use to help them remember what a room is suppose to be and for which team. These indicators are the red and blue signs with simple text. Instead of these, you should use a prop_static intelligence sign.
Fundamentally, there is nothing "wrong" with the map, as it's solidly built and I only found one instance of texture mishap. There is an overlapping texture near the top entrance going into blue's base on the tunnel wall itself. This should be very easy to fix. But it could use some sprucing up. First, the lighting in the tunnels may be a bit harsh. Perhaps, you could use a different color other than white--maybe a light gray or a whitish brown. This would tone down the harshness without making it too dark.
Also, some areas really could benefit from being dark like the side rooms and the stairways in the tunnels. This allows for some classes to have an advangtage with ambushing. Having said that, I think some of the tunnels need to be wider, taller, and have more objects of cover. Right now, spies and pyros are going to have a heck of a time trying to get around behind the enemy. If there were larger areas to move around in with greater cover, these classes would benefit.
One other major flaw is the exclusion of player clips. Right now the walls of the tunnels and the support beams have no player clips, and you will get stuck quite often while running through the tunnels. These really should be clipped so you don't have to worry about getting stuck and losing forward progress. This is one of the major things to keep in mind for TF2 map design. But this would be simple enough to add.
Other than these key points, I really like the map, especially knowing it's your first. Keep up the work, and I hope you take my advice in some of these areas. Also, as Mr. White said, contact me if you want to setup a map test on the TF2F servers.